Context. M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in
January 2015 – the first event of this kind observed in M 31 since 1988. Very few similar
events have been confirmed as of 2015. Most of them are considered to be products of
stellar mergers.

Aims. Results of an extensive optical monitoring of the transient in the
period January–March 2015 are presented.

Methods. Eight optical telescopes were used for imaging. Spectra were
obtained on the Large Altazimuth Telescope (BTA), the Gran Telecsopio Canarias (GTC) and
the Rozhen 2 m telescope.

Results. We present a highly accurate 70 d light curve and astrometry
with a 0.05′′uncertainty. The
colour indices reached a minimum of 2−3 d before peak brightness and rapidly increased afterwards. The
spectral type changed from F5I to F0I in 6 d before the maximum and then to K3I in the
next 30 d. The luminosity of the transient was estimated to be
8.7+3.3-2.2 ×
105L⊙ during the optical
maximum.

Conclusions. Both the photometric and the spectroscopic results confirm
that the object is a red nova, similar to V838 Monocerotis.

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